Mems switches with reduced switching voltage and methods of manufacture

ABSTRACT

MEMS switches and methods of manufacturing MEMS switches is provided. The MEMS switch having at least two cantilevered electrodes having ends which overlap and which are structured and operable to contact one another upon an application of a voltage by at least one fixed electrode.

FIELD OF THE INVENTION

The invention relates to MEMS switches and methods of manufacturing MEMSswitches and, more particularly, MEMS switches with reduced switchingvoltage and methods of manufacture.

BACKGROUND

Integrated circuit switches used in 3D and other integrated circuits canbe formed from solid state structures (e.g., transistors) or passivewires (MEMS). MEMS switches are typically employed because of theiralmost ideal isolation, which is a critical requirement for wirelessradio applications where they are used for mode switching of poweramplifiers (PAs).

MEMS can be manufactured in a number of ways using a number of differenttools. In general, though, the methodologies and tools are used to formsmall structures with dimensions in the micrometer scale. Also, many ofthe methodologies, i.e., technologies, employed to manufacture MEMS havebeen adopted from integrated circuit (IC) technology. For example,almost all MEMS are built on wafers and are realized in thin films ofmaterials patterned by photolithographic processes. More specifically,the fabrication of MEMS use three basic building blocks: (i) depositionof thin films of material on a substrate, (ii) applying a patterned maskon top of the films by photolithographic imaging, and (iii) etching thefilms selectively to the mask.

Depending on the particular application and engineering criteria, MEMSstructures can come in many different forms. For example, MEMS can berealized in the form of a single cantilever structure such as, forexample, shown in U.S. Pat. No. 7,265,492. In this cantileverapplication, a single cantilever arm (suspended electrode) is pulledtoward a fixed electrode by application of a voltage. In knownapplications, the voltage required to pull the suspended electrode downto the fixed electrode by electrostatic force may be high, which hasbeen seen to cause unwanted charging on insulator after prolonged useand eventual failure of the switch. In certain applications, the highvoltage, e.g., 100 volts, is also difficult to obtain since this has tobe stepped up from about 1.5 volts to about 5 volts. The minimum voltagerequired is called pull-in voltage, which is dependent on area of theelectrode, spacing or gap between the suspended and fixed electrodes,and spring constant of the membrane or springs.

Lowering the pull-in voltage without decreasing the gap and withoutsoftening the spring is desirable, as the spring provides the restoringforce and determines the switching speed. In U.S. Pat. No. 7,265,492, apair of side parallel-plate electrostatic actuators is implemented forlowering or eliminating of the bias voltages. These additionalelectrostatic actuators are used to reduce or eliminate the bias voltageto be applied on the fixed signal electrode. In implementation, thefixed electrode of the side parallel-plate electrostatic actuators canbe elevated above a fixed signal electrode. Thus due to a smaller gap,the pull-in voltage required to pull the suspended electrode down to thefixed electrode can be lowered. However, the MEMS shown in U.S. Pat. No.7,265,492 are not hermetically sealed, and the additional electrostaticactuators can increase fabrication costs.

Accordingly, there exists a need in the art to overcome the deficienciesand limitations described hereinabove.

SUMMARY

In a first aspect of the invention, a structure comprises at least twocantilevered electrodes having ends which overlap and which arestructured and operable to contact one another upon an application of avoltage by at least one fixed electrode.

In a second aspect of the invention, a method of fabricating a switchcomprises forming at least two cantilever electrodes and at least onefixed electrode through a series of resist deposition and patterningsteps.

In yet another aspect of the invention, a method of forming a switchcomprises: depositing layers of resist on a structure; patterning theresist to form sequential openings; sequentially depositing metal ormetal alloy within the sequential openings until at least two cantileverelectrodes and at least one voltage applying electrode are formed withinthe layers of resist; depositing a liner over an uppermost layer of thelayers of resist; forming openings in the liner; etching the layers ofthe resist through the opening until the cantilever electrodes and theat least one voltage applying electrode are in a void; and sealing thevoid with additional liner material to form a hermetically sealed dome.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is described in the detailed description whichfollows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the present invention,in which:

FIGS. 1 a-1 i show intermediate structures and respective fabricationprocesses in accordance with the invention;

FIG. 2 shows a MEMS structure in accordance with a first aspect of theinvention;

FIG. 3 shows a MEMS structure in accordance with a second aspect of theinvention;

FIG. 4 shows a MEMS structure in accordance with a third aspect of theinvention;

FIG. 5 shows a MEMS structure in accordance with a fourth aspect of theinvention; and

FIG. 6 shows a MEMS structure in accordance with a fifth aspect of theinvention.

DETAILED DESCRIPTION

The invention relates to MEMS switches and methods of manufacturing MEMSswitches and, more particularly, MEMS switches with reduced switchingvoltage and methods of manufacture. In implementation, the inventionincludes methods and structures of several novel MEMS switches optimizedfor (1) switching voltage (i.e. reducing it) and (2) reliability.

The MEMS switches of the invention include at least a double cantileverarrangement hermetically sealed within a nitride type liner, forexample. In operation, a gap between the electrodes is reduced, comparedto conventional MEMS switches. This arrangement will reduce the minimumswitching voltage required to pull the electrodes together (i.e.,reduced pull-in voltage) and/or the on time of the voltage. Accordingly,unwanted charging on insulator and failure of the switch can be reduced,compared to known conventional switches. Also, in operation, the MEMSswitches substantially eliminate arcing, as well as large dielectricbreakdown attributable to higher switching voltages. Although the MEMSswitches are shown with a nitride hermetic seal, MEMS switchesfabricated using the same methodology, either without nitride heremeticseals or with other methods of hermetic seals, such as a MEMS switchinside a cavity with a bonded chip or wafer cap, are contemplated by thepresent invention.

Exemplary Fabrication Processes in Accordance with the Invention

FIGS. 1 a-1 i show intermediate structures and respective fabricationprocesses in accordance with the invention. More specifically, theprocesses shown and described with reference to FIGS. 1 a-1 h aredirected to the embodiment of FIG. 2. However, it should be recognizedby those of skill in the art that with some modifications and/oradditions to the processes described herein, e.g., patterning,metallization and/or deposition processes, the processes of FIGS. 1 a-1i can be used to fabricate any of the embodiments described herein.Although such modifications and/or additions should become obvious tothose of skill in the art after an explanation of each of theembodiments, some further explanation of the additional and/or modifiedprocesses are described herein as necessary for a more thoroughunderstanding of the invention.

More specifically, FIG. 1 a shows a beginning structure in accordancewith the invention. The beginning structure includes a dielectricmaterial 10 with a plurality of vias 12. As should be understood bythose of skill in the art, the dielectric material 10 may be an M+1wiring layer in an integrated circuit. Although not shown in FIG. 1, itshould be understood that the dielectric material 10 may be provided ona wafer of any known type used with the formation of integratedcircuits. For example, the wafer can be silicon, BULK, SOI, SiGe;quartz; glass; or Gallium arsenide, to name a few. The vias 12 can bemetallized using any combination of methods known in the art, such asphysical vapor deposition (PVD), chemical vapor deposition (CVD),electroplated deposition (ECP), metal-organo chemical vapor deposition(MOCVD), etc. In one exemplary embodiment, the vias are tungsten plugs,with TiN liners. In another embodiment, the vias are formed using copperwith TaN/Ta liners. In another embodiment, the vias are ‘tapered vias’which are metallized with the conductor layer used to form 16 a and 18 ashown in FIG. 1 b.

The plurality of vias 12 are formed using conventional lithographicprocesses. For example, a resist is deposited on the dielectric material10 and selective portions of the resist are exposed to form openings. Insubsequent processes, the dielectric material 10 is etched using aconventional process such as, for example, reactive ion etching (RIE) toform vias. The vias are filled with known metals or metal alloys to formthe vias 12. The resist can be stripped away. The vias 12 can act asconductive pads as noted in more detail below.

In FIG. 1 b, the lower conductive MEMS switch electrodes are formed.These can be formed using any known method, such as by depositing theconductor, lithographically patterning it, etching it, and removing thephotoresist used for lithographic patterning. Alternatively, other knownmethods, such as lift-off or damascene could be used. FIG. 1 b shows adamascene method in which a sacrificial resist layer 14 is depositedover the structure of FIG. 1 a. In one embodiment, the sacrificialresist layer 14 comprises Polymethylglutarimide (PMGI). PMGI iscompatible with most g-line, i-line, and DUV photoresists and hasexcellent adhesion to Si, SiN, NiFe, Cu, Au, GaAs, and otherIII-V/III-VI materials. PMGI also exhibits a high thermal stability andcan be applied in any conventional manner such as, for example,spin-coating. The PMGI can be stripped in NMP and DMSO-based removers.Also, PMGI is DUV, E-beam, and x-ray sensitivity, as well as exhibits ahigh etch rate in oxygen plasma.

In conventional processes, the sacrificial resist layer 14 is patternedto form openings. The openings are filled with a metal such as gold;although, other metals or metal alloys are also contemplated by theinvention such as AlCu, W, or Cu. Prior to the deposition of the metal,one or more refractory metals, such as Ti, TiN, Ta, TaN, Ru, etc. can beused to line the vias. In the embodiment described herein, the metalwill form fixed electrodes 16 a and 16 b and cantilevered electrodes 18a and 18 b.

In processing steps shown in FIGS. 1 c-1 f, additional deposition andpatterning processes are sequentially shown to build the fixedelectrodes 16 a and 16 b and cantilevered electrodes 18 a and 18 b. Thedeposition and patterning processes are similar to that described withreference to FIG. 1 b and, as such, additional explanation is notrequired for a complete understanding of the invention. In embodiments,the processes described herein result in the arm (beam) of thecantilever electrode 18 b being formed in the processes shown in FIG. 1d and the arm of the cantilever electrode 18 a being formed in theprocesses shown in FIG. 1 f. Also, as shown in FIG. 1 f, the electrodes18 a and 18 b have respective end portions 18 a ₁ and 18 b ₂ thatoverlap. The overlapping end portions 18 a ₁ and 18 b ₂, uponapplication of a voltage, will close the switch, as discussed in greaterdetail below.

As shown in FIG. 1 g, a hermetic dielectric, such as Si₃N₄ (nitride)liner 20 is deposited over the structure of FIG. 1 h. In embodiments,the liner 20 can be SiN. In FIG. 1 h, holes 22 are etched into the liner20 to form openings, exposing the sacrificial resist. In subsequentprocesses, a wet etching using, for example, NMP (N-methylpyrrolidone)is used to dissolve the sacrificial resist encapsulated within the liner20, creating a void 24. The void 24 is hermetically sealed by adeposition of Nitride 23 in order to close the holes 22 and form ahermetically sealed dome, as shown in FIG. 1 i. As discussed above, thenitride dome is used to hermetically seal the MEMS switch and isoptional. Although the method used for forming the freestandingcantilever beams uses the sacrificial PMGI resist, any known method offorming cantilever beams, such as using sacrificial α-silicon,subtractive-etch wiring, tapered via wiring, etc. could also be used.

First Aspect of the Invention

FIG. 2 shows a MEMS structure in accordance with a first aspect of theinvention. In this aspect of the invention, the electrodes 16 a and 16 band 18 a and 18 b are hermetically sealed within the nitride layer 25.The nitride dome 25 can be oval shaped, as shown in FIG. 2, rectangularshaped, as shown in FIG. 1, or any other known shape. In embodiments,the arms of the cantilever electrodes 18 a and 18 b are about 50 micronslong, 9 microns high and 20 microns long; although other dimensions arealso contemplated by the invention. Also, in further embodiments, adistance “X” between the respective overlapping end portions 18 a ₁ and18 b ₂ is about two microns; although, other distances are alsocontemplated by the invention. To decrease the distance “X”, verticallyextending portions (as shown in the embodiment of FIG. 5) can extendfrom each of the cantilever electrodes 18 a and 18 b.

In operation, upon the application of a positive voltage to the fixedelectrode 16 a, the cantilever electrode 18 a will be pulled downtowards the cantilever electrode 18 b. Also, upon a negative voltageapplied to the fixed electrode 16 b, the cantilever electrode 18 b willbe pushed up towards the cantilever electrode 18 a. At a predetermineddesigned voltage, the respective overlapping end portions 18 a ₁ and 18b ₂ will make contact with one another, i.e., travel the distance “X”,thereby closing the switch. In the off state (0 voltage), the cantileverelectrodes 18 a and 18 b will return to their original position, with aspace “X” between the respective ends.

In one design, the voltage applied to the fixed electrode 16 a is about30 volts and the fixed electrode 16 b is about −30 volts. This designvoltage can be significantly lower than known conventional systems asthe two cantilever arms are each designed and arranged to move a smallerdistance than a single arm in a conventional system. More specifically,there can be a reduced switching voltage due to bending of both arms andthe use of two voltage electrodes (e.g., 16 a and 16 b).

This reduced voltage is a minimum switching voltage required to pull theelectrodes together (i.e., pull-in voltage). This reduced voltage canresult in many advantages such as, for example, reduced on time voltage,unwanted charging on insulator and reduced failure of the switch(compared to known conventional switches). Also, in operation, the MEMSswitch of FIG. 2 substantially eliminates arcing, as well as largedielectric breakdown attributable to higher switching voltages.

Second Aspect of the Invention

FIG. 3 shows a MEMS structure in accordance with a second aspect of theinvention. In this aspect of the invention, the fixed electrode 16 b hasbeen eliminated. As in the other embodiments, the fixed electrode 16 aand the cantilever electrodes 18 a and 18 b are hermetically sealedwithin the nitride layer 22. In embodiments, the arms of the cantileverelectrodes 18 a and 18 b are about 50 microns long, 9 microns high and20 microns long; although other dimensions are also contemplated by theinvention. Also, in further embodiments, a distance “X” between theoverlapping respective end portions 18 a ₁ and 18 b ₁ is about twomicrons; although, other distances are also contemplated by theinvention. To decrease the distance “X”, vertically extending portions(as shown in the embodiment of FIG. 5) can extend from one or both ofthe cantilever electrodes 18 a and 18 b.

In operation, upon the application of a positive voltage to the fixedelectrode 16 a, the cantilever electrode 18 a will be pulled downtowards the cantilever electrode 18 b. At a predetermined designedvoltage, the respective overlapping end portions 18 a ₁ and 18 b ₁ willmake contact with one another, i.e., travel the distance “X”, therebyclosing the switch. In one design, the voltage applied to the fixedelectrode 16 a is about 100 volts. In the off state (0 voltage), thecantilever electrode 18 a will return to its original position, with aspace “X” between the respective end portions 18 a ₁ and 18 b ₂. In thisembodiment, the cantilever electrode 18 b is designed to remainstationary.

This arrangement also provides advantages such as, for example, reducedsticktion of the electrodes 18 a and 18 b. More specifically, as thereare two cantilever arms 18 a and 18 b, it is theorized that that switchwill stayed in the closed position, upon the application of a voltage,better than conventional MEMS switches. This will ensure that the switchwill not fail.

Third Aspect of the Invention

FIG. 4 shows a MEMS structure in accordance with a third aspect of theinvention. In this aspect of the invention, the electrodes 16 a and 16 band 18 a and 18 b are hermetically sealed within the nitride layer 22.In embodiments, the arms of the cantilever electrodes 18 a and 18 b havedifferent lengths, such that 18 b does not extent over 16 a, where thearm of the cantilever electrode 18 a is longer than the arm of thecantilever electrode 18 b (although this can be reversed). To decreasethe distance “X”, vertically extending portions (as shown in theembodiment of FIG. 5) can extend from one or both of the cantileverelectrodes 18 a and 18 b.

In embodiments, the arm of the cantilever electrode 18 a extends overboth of the fixed electrodes 16 a and 16 b. Also, in the embodiment ofFIG. 4, the cantilever electrode 18 b is fixed, i.e., embedded, to thenitride layer 22. This can be achieved by depositing the nitride layer22 directly onto an upper surface of the cantilever electrode 18 b. Inan alternative embodiment, the cantilever electrode 18 b can also befloating by adding an upper layer of sacrificial material (PMGI) priorto the deposition of the nitride layer 22. Also, in further embodiments,a distance “X” between the overlapping cantilevers 18 a and 18 b isabout two microns; although, other distances are also contemplated bythe invention. To decrease the distance “X”, vertically extendingportions (as shown in the embodiment of FIG. 5) can extend from thecantilever electrode 18 a or the fixed electrode 16 b.

In operation, upon the application of a positive voltage to the fixedelectrode 16 a, the cantilever electrode 18 a will be pulled downtowards the fixed electrode 16 b. Also, upon a negative voltage appliedto the cantilever electrode 18 b, the cantilever electrode 18 a will bepushed down towards the fixed electrode 16 b. At a predetermineddesigned voltage, the end portion 18 a ₁ will make contact with thefixed electrode 16 b, i.e., travel the distance “Y”, thereby closing theswitch. In the off state (0 voltage), the cantilever electrode 18 a willreturn to its original position, with a space “Y” between the cantileverelectrode 18 a and the fixed electrode 16 b. In this design, thecantilever electrode 18 b remains stationary, as it is fixed to thenitride liner 22.

In one design, the voltage applied to the fixed electrode 16 a is about50 volts and the voltage applied to the cantilever electrode 18 b isabout −50 volts. This design voltage can be significantly lower thanknown conventional systems as the arm of the cantilever electrode 18 ais being pushed and pulled by the use of two voltage electrodes (e.g.,16 a and 18 b).

This reduced voltage is a minimum switching voltage required to pull theelectrodes together (i.e., pull-in voltage). This reduced voltage canresult in many advantages such as, for example, reduced on time voltage,unwanted charging on insulator and reduced failure of the switch(compared to known conventional switches). Also, in operation, the MEMSswitch of FIG. 4 substantially eliminates arcing, as well as largedielectric breakdown attributable to higher switching voltages.

Fourth Aspect of the Invention

FIG. 5 shows a MEMS structure in accordance with a fourth aspect of theinvention. In this aspect of the invention, an additional cantileverelectrode 18 c is formed using the processes described above, e.g.,adding an additional deposition layer of resist and additionalpatterning steps. The electrodes 16 a, 16 b and 18 a-18 c arehermetically sealed within the nitride layer 22. In embodiments, the armof the cantilever electrode 18 a also includes a vertical extendingprotrusion (nub) 18 a ₂. As in other embodiments, the cantileverelectrodes 18 a and 18 b are about 50 microns long, 9 microns high and20 microns long; although other dimensions are also contemplated by theinvention. Also, in embodiments, a distance “X” between the respectiveportions 18 a ₂ and 18 b ₁ is about two microns; although, otherdistances are also contemplated by the invention. To decrease thedistance “X”, vertically extending portions can extend from each of thecantilever electrodes 18 a and 18 b.

In operation, upon the application of a positive voltage to the fixedelectrode 16 b, the cantilever electrode 18 b will be pulled downtowards the cantilever electrode 18 a. Also, upon a negative voltageapplied to the fixed electrode 16 a and a positive voltage applied tothe cantilever electrode 18 c, the cantilever electrode 18 a will bepushed upwards toward the cantilever electrode 18 b. At a predetermineddesigned voltage, the respective nub portion 18 a ₂ and the end portion18 b ₁ will make contact with one another, i.e., travel the distance“X”, thereby closing the switch. In the off state (0 voltage), thecantilever electrodes 18 a and 18 b will return to their originalposition, with a space “X” between the respective ends.

In one design, the voltage applied to the fixed electrode 16 b and thecantilever electrode 18 c is about 30 volts. Also, the voltage appliedto the fixed electrode 16 a is about −30 volts. This design voltage canbe significantly lower than known conventional systems as the twocantilever arms (18 a and 18 b) are each designed and arranged to move asmaller distance than a single arm in a conventional system. Morespecifically, there can be a reduced switching voltage due to bending ofboth arms and the use of three voltage electrodes (e.g., 16 a, 16 b and18 c).

This reduced voltage is a minimum switching voltage required to pull theelectrodes together (i.e., pull-in voltage). This reduced voltage canresult in many advantages such as, for example, reduced on time voltage,unwanted charging on insulator and reduced failure of the switch(compared to known conventional switches). Also, in operation, the MEMSswitch of FIG. 5 substantially eliminates arcing, as well as largedielectric breakdown attributable to higher switching voltages.

Fifth Aspect of the Invention

FIG. 6 shows a MEMS structure in accordance with a fifth aspect of theinvention. In this aspect of the invention, an additional cantileverelectrode 18 c is formed using the processes described above, e.g.,adding an additional deposition layer of resist and additionalpatterning steps. Also, the fixed electrode 16 b can be raised to becloser to the cantilever electrode 18 b which, in turn, provides animproved response time (as the space between the fixed electrode 16 band the cantilever electrode 18 b is closed). The height of the fixedelectrode 16 b is higher than the height of the fixed electrode 16 a. Todecrease the distance “X”, a vertically extending portion can alsoextend from the cantilever electrodes 18 b. A gap is approximately 5 to10 microns between the electrode 16 b and the cantilever electrode 18 b.In embodiments, the electrodes 16 a, 16 b and 18 a-18 c are hermeticallysealed within the nitride layer 22.

In embodiments, the arm of the cantilever electrode 18 a also includes avertical extending protrusion (nub) 18 a ₂. As in other embodiments, thecantilever electrodes 18 a and 18 b are about 50 microns long, 9 micronshigh and 20 microns long; although other dimensions are alsocontemplated by the invention. Also, in further embodiments, a distance“X” between the respective end portions 18 a ₁ and 18 b ₂ that overlapis about two microns; although, other distances are also contemplated bythe invention.

In operation, upon the application of a positive voltage to the fixedelectrode 16 b, the cantilever electrode 18 b will be pulled downtowards the cantilever electrode 18 a. Also, upon a negative voltageapplied to the fixed electrode 16 a and a positive voltage applied tothe cantilever electrode 18 c, the cantilever electrode 18 a will bepushed upwards toward the cantilever electrode 18 b. At a predetermineddesigned voltage, the respective nub portion 18 a ₂ and the end portion18 b ₁ will make contact with one another, i.e., travel the distance“X”, thereby closing the switch. In the off state (0 voltage), thecantilever electrodes 18 a and 18 b will return to their originalposition, with a space “X” between the respective ends.

In one design, the voltage applied to the fixed electrode 16 b and thecantilever electrode 18 c is about 30 volts. Also, the voltage appliedto the fixed electrode 16 a is about −30 volts. This design voltage canbe significantly lower than known conventional systems as the twocantilever arms (18 a and 18 b) are each designed and arranged to move asmaller distance, than a single arm in a conventional system. Morespecifically, there can be a reduced switching voltage due to bending ofboth arms and the use of three voltage electrodes (e.g., 16 a, 16 b and18 c).

This reduced voltage is a minimum switching voltage required to pull theelectrodes together (i.e., pull-in voltage). This reduced voltage canresult in many advantages such as, for example, reduced on time voltage,unwanted charging on insulator and reduced failure of the switch(compared to known conventional switches). Also, in operation, the MEMSswitch of FIG. 6 substantially eliminates arcing, as well as largedielectric breakdown attributable to higher switching voltages.

The structures as described above are used in the fabrication ofintegrated circuit chips. The resulting integrated circuit chips can bedistributed by the fabricator in raw wafer form (that is, as a singlewafer that has multiple unpackaged chips), as a bare die, or in apackaged form. In the latter case the chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multichip package(such as a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case the chip isthen integrated with other chips, discrete circuit elements, and/orother signal processing devices as part of either (a) an intermediateproduct, such as a motherboard, or (b) an end product. The end productcan be any product that includes integrated circuit chips, ranging fromtoys and other low-end applications to advanced computer products havinga display, a keyboard or other input device, and a central processor.

While the invention has been described in terms of exemplaryembodiments, those skilled in the art will recognize that the inventioncan be practiced with modifications and in the spirit and scope of theappended claims.

It is claimed:
 1. A method of fabricating a switch comprising: forming afixed electrode; forming a first cantilevered electrode; forming asecond cantilevered electrode aligned vertically over the fixedelectrode, and which has an end that overlaps and is operable todirectly contact an end of the first cantilevered electrode upon anapplication of a voltage to the fixed electrode; and forming ahermetically sealed volume encapsulating the fixed electrode, the firstcantilevered electrode, and the second cantilevered electrode.
 2. Themethod of claim 1, wherein the forming the hermetically sealed volumecomprises: forming at least one hole in a nitride liner to gain accessto a sacrificial resist; etching the sacrificial resist; and closing theat least one hole by deposition of additional liner material.
 3. Themethod of claim 1, wherein the hermetically sealed volume is a dome. 4.The method of claim 3, wherein the dome is oval shaped.
 5. The method ofclaim 1, wherein overlapping portions of the first cantileveredelectrode and the second cantilevered electrode are separated by avertical distance of about two microns.
 6. The method of claim 1,wherein the second cantilevered electrode is formed above the firstcantilevered electrode.